In connection with sulfate, sulfite and other cooking processes, the delignification of lignocellulose material, such as wood chips, is carried out. Thus, the lignocellulose material is cooked in a cooking liquor under predetermined conditions of time, temperature, pressure, the supply of chemicals, and the like. When the cooking has been concluded, conditions of high pressure and temperature prevail in the digester.
Discharge of the digester can be performed by opening a valve at the bottom of the digester so that the contents of the digester are blown out by means of the pressure prevailing within the digester through a conduit into a receiving vessel which is maintained at atmospheric pressure. Alternatively, the contents of the digester can be cooled by displacing the hot spent cooking liquor within the digester with cooler waste liquor.
In these processes, the contents of the digester are then blown out by means of high pressure air or steam into the receiving vessel. These processes are known as the "cold blow" processes. They are exemplified, for example, by Canadian Patent No. 1,135,101, corresponding to Swedish Patent No. 435,075; and by European Patent No. 100,293. In the Canadian '101 patent a system is provided for blowing the pulp from the digester at relatively low temperatures, preferably in the range of from 90.degree. to 105.degree. C., and in which the pulp may be prewashed within the digester. In particular, as is shown in connection with the discussion of FIG. 3 therein, at the final stage of digesting, the cooking liquor is fed to an equalizing tank maintained under pressure. The pressure in the digester 1 is then lowered to substantially atmospheric pressure by opening a steam valve, and washing liquor is fed to the digester during this period through valve 15 as shown in FIG. 3. In this manner, the contents of the digester are blown at the above-mentioned temperatures to provide such a cold blow process.
As for the European '293 patent, a further such cold blow process is shown with respect to FIG. 1 thereof in which the spent cooking liquor is discharged from the digester by using a curtain of compressed, relatively cool air at the top of the digester to force the pulp out through a blow valve into a blow tank. In accordance with this process, washer filtrate liquor from tank 17 is used as the displacement liquid during operation of the digester. It is at this point that the relatively cool air is injected into the top of the digester, i.e., after displacement, preferably at a pressure of about 0.5 to 1.0 MPa.
In both of these above cold blow processes, flushing or displacement is required therewith. Also, high pressure is required in order to achieve blowing from the digester in order to guarantee complete discharge of the material therein. However, at the same time, discharge is difficult to control since the consistency of the material will vary considerably. Furthermore, steam or air can penetrate the material through channels prior to emptying of the digester.
Additional problems in these prior art systems concern control of possible blow condensers. Thus, problems with potential pressure shocks to gas treatment and with transfer to condensate can arise. In order to deal with these problems, the gas treatment equipment has thus been extensive and quite costly.
One method of obtaining a more controlled discharge is to relieve the overpressure in the digester, and to reduce the temperature to below about 100.degree. C. The material can then be pumped out of the digester by means of a pump. In this manner, the need for gas treatment equipment can also be reduced or avoided. This method is known for use with batch digesters for sulfite, sulfate and other types of cooking.
However, the use of pump discharge has a number of concomitant disadvantages. A pump is very sensitive to the pressure of coarse particles. These raw materials, however, are normally accompanied by different metal objects, stone and concrete clods, as well as large pieces of wood. Even if very large pump wheels are thus used, there remains a considerable risk of sticking and clashing taking place, with resultant disturbances in production. Furthermore, pump discharge can also result in increased servicing costs and decreased reliability in operation compared with the conventional blow discharge mentioned above. In order to reconstruct old digesters, problems of sufficient space for pumps and associated conduits can arise, and thus the installation can be unreasonably high.
Another method of discharging a digester is to install a rotating discharge device. In this manner, the total digester pressure is used as the discharge force. Uniform discharge is obtained in this case by dilution of the material in a dilution zone, with simultaneous stirring by means of the discharge device itself. Such an arrangement is possible in connection with a continuous digester having a bulging bottom and employing continuous discharge. However, with a batch digester, which is normally provided with a conical bottom, such a discharge device is not suitable. Moreover, the installation and servicing of such devices in batch digesters would require considerable costs, particularly with respect to the large number of such digesters in a plant.